Shunt for oil-immersed circuit breaker

ABSTRACT

The shunt of an oil-immersed circuit breaker is made of porous refractory conglomerate so that it becomes saturated with oil, whereby on passage of the high electric currents experienced with circuit breakers, the oil is rapidly heated and ejected from the pores to carry away energy at a high rate.

United States Patent Adwick et al. 1451 Aug. 29, 1972 [54] SHUNT FOROIL-IMMERSED CIRCUIT BREAKER [56] References Cited [7 Inventors: g g gg; UNITED STATES PATENTS t 1 fi g S 1,509,495 9/1924 Slepian ..338/21 x2,305,577 12/1942 Stoelting ..338/21 Asslgnem Morganite Resistors iited, J 2,281,385 4/1942 Saint-Germain et al.200/146R row, England [22]1970 Primary Examiner-C. L. Albritton [21] Appl. N0.: 82,683AttorneyLarson, Taylor & Hinds [30] Foreign Application Priority Data[57] I ABS CT The shunt of an oil-immersed circuit breaker is made 1969Great Bmam "52l45/69 of porous refractory conglomerate so that itbecomes saturated with oil, whereby on passage of the high [52] US. Cl...338/49, 200/146 R electric currents experienced with circuit breakers,[51] Till. Cl. ..H01C 13/09 the oil is rapidly heated and ejected f hpores to [58] Field of Search ..338/20, 21, 49, 223-225, carry awayenergy at a high rate 317/9 R, 11 C, 11 E, 60', 200/144 R, 146 A, 146RSCIaimsADraWingFigui-es Patented Aug. 29,, 1972 2 Sheets-Sheet 1 FIG]Patentcd Aug. 29, 1972 y 3,688,242

- 2 Shuts-Sheet 2 DESCRIPTION This invention relates to electricalresistance elements suitable for use in avoiding practical difficultiesexperienced with certain electrical equipment notably circuit breakers.

When an electric current is interrupted by a circuit breaker an arc mayform between the separating contacts prolonging the current interruptingprocess. However, with high voltage apparatus, unless this arc occursvery high induced currents develop in inductive circuits associated withthe circuit breaker. When circuit breakers are used in association withhigh voltage/high current apparatus, a very considerable quantity ofenergy is dissipated at each operation of the circuit breaker and itsdissipation must be carefully controlled to avoid explosions or fires ordamage to the apparatus. Two common methods of arc control are blowingout the arc with a blast of cold air and immersing the contacts in oilwhich has a high dielectric constant and can carry away heat energy.

Current control in circuit breakers can be assisted by connecting aresistor in shunt across the circuit breaker contacts and this inventionprovides an improved form of such shunt suitable for use withoil-immersed circuit breakers.

The improved shunt is formed so as to be permeable to a fluid so that onpassage through the shunt of an electric current, the fluid with whichthe shunt is permeated is heated, and the shunt consists of a refractoryconglomerate of at least two materials, respectively of a high and a lowelectrical resistivity, forming an open cellular structure having anapparent porosity not less than 25 percent and having a selected poresize.

In the use of such a fluid-permeable electrical resistance element as ashunt in an oil-immersed circuit breaker, the oil, which permeates andfills the pores, is rapidly heated by current passing through theelement during circuit breaking and is expelled from the pores therebyrapidly carrying energy away from the element. Also, as is usual inoil-immersed circuit breakers, the oil, which has a high dielectricconstant, helps to inhibit striking and restriking of an arc, forinstance such as might occur between points formed by carbon particleson the surface of the resistance element.

The resistance element of this invention to be utilized as a shuntshould have a total porosity of at least 25 percent and preferablybetween 25 and 30 percent, and the pores should be of a selectedcross-sectional area such as to avoid disruptive failure on injection ofsuch a large quantity of energy as would occur in circuit breaking. Forinstance, since the energy injection could be more than 2,000 joules/cc,the temperature of the element could be increased to about l,0 C. in 100milliseconds. Disruptive failure might occur due to high internalpressures developed due to such rapid heating of the oil if the poresare too small.

Also with most ceramics the pores should not be too large since thiscould lead to inferior performance either mechanically owing to thereduction in the number of bonds between granules, or electrically owingto surface tracking at lower voltages.

It is found that if the mean theoretical pore diameter is of the orderof 1.3 1- 0.1 microns for a porosity of the element of between 25percent and 30 percent, excessive pressure build-up in the pores isavoided. Such a pore size compares with a mean pore diameter of about1.0 micron for a normal ceramic resistor.

The pores are merely required to be such as to enable the element tobecome oil saturated and to permit the oil to be expelled as abovedescribed when the circuit breaker operates. Thus the pores need not,but may, extend between two surfaces of the element.

A resistance element in accordance with the invention may be madelargely by techniques known for making clay-carbon and similarelectrical resistors except that care must be taken to avoid suchvitrification of the material that its permeability is impaired.

In general, care should be taken to avoid the use of constituents,particularly binders, which vitrify on heating, a substantial proportionof relatively large particles of the basic refractory material should beincluded and only moderate pressures and a temporary binder, i.e. avolatile or combustible binder, should be used in forming the element'to shape before firing.

In the manufacture of resistance elements of this invention, the poresize and permeability control can be effected by selecting the grainsize of the mix of the conglomerate from which the shape is produced andcarefully pressing the shape prior to firing.

Suitable clay-carbon conglomerates have the following generalcomposition, by weight of starting materials, the proportions beingvaried to suit the electrical and fluid permeability requirements.

The quantity of carbon used in the mix is selected to give the desiredresistivity.

Methods of production of fluid-permeable electrical resistance elementsof this invention will now be described in more detail.

- The materials are thoroughly mixed, with or without water according tothe kind of element to be made, formed to shape, such as bydry-pressing, mouldingor extrusion and then fired, preferably betweenl,200 and 1,400 C., in a non-oxidizing or reducing atmosphere. Mixingmay be effected in any conventional ceramic mixer, e.g. a Planetarymixer or Sigma Blade mixer.

In the case of dry-pressing, a temporary binder, such as 3 percent w.w.ethylene glycol, may be used.

EXAMPLE Ceramic fillers (equal parts of calcined china clay and alumina)Clay binder (equal parts of ball clay and bentonite) Carbon The ceramicfillers have a particle size up to about microns diameter, the clayabout 1 micron diameter and the carbon about 0.1 micron diameter. Theceramic ingredients are made up into a paste with water and the carbonadded as a slip.

The mix is dried to between 6 and 7 percent moisture content asdetermined by a Speedy Moisture Tester."

The mix so produced is next granulated by being forced through a sieveof selected mesh, e.g. 16, 18 or 22 B.S.S. mesh, and the resultingparticles sieved on a smaller mesh sieve, e.g. 25, 30 or 36 B.S.S. mesh.The latter step may be omitted if the first sieving is carried out atsuch a speed that the particle size is about 0.50 mm and there is littledistribution of particle size.

The sieve aperture sizes corresponding to the above B.S.S. mesh numbersare given in following table:

The shapes to be fired are produced by pressing the granular material inpunch dies to give a density of 2.35 1*: 0.05 gm/cm corresponding to afired density of about 2.2 gm/cm.

The pressed shapes are then fired. Conveniently, for firing, the shapesare packed in saggars containing loose carbon material, e.g. graphite orbroken coke, which combines with any oxygen in the furnace to provide aninert atmosphere. The temperature of firing is between 1,200 and 1,400C., the temperature employed determining the degree of sintering.

Finally, the surfaces are metallized as required. Also the fired shapemay have its peripheral surface treated with an anti-tracking coating.

In tests, using a resistance element having a resistivity of 250!) inchpressed from particles pressed through an 18 B.S.S. mesh sieve andretained on a 30 B.S.S. mesh sieve and pressed to a density of 2.37gm/cm the following results were obtained:

Time of voltage duration at 50H: (milliseconds) Applied voltage at discfailure kV(rms) per inch The stability after energy injection was asfollows:

Energy The elements, which are conveniently cylindrical, may be 3 inchesin outside diameter, and 1 inch long with a l-inch diameter bore and inuse a number of these may be threaded on to an insulating rod to give aresistor value of 15 ohms for circuit breakers in 5 to 6 kV supplies.

This invention therefore comprises the production of fluid permeableelectrical resistance elements having a mean theoretical pore diameterof the order of 1.3 microns for a porosity of 25 to 30 percent, andcomprises also the combination of such elements with oilfilled circuitbreakers in shunt across the circuit breaker contacts.

The accompanying drawings illustrate by way of example an oil-filledcircuit breaker and one form of shunt of this invention.

In the drawings,

FIG. 1 is a diagrammatic section through the circuit breaker,

FIGS. 2 and 3 shown in plan and elevation a typical shunt element, and

FIG. 4 shows a shunt such as may be used in a circuit breaker as shownin FIG. 1.

Referring to FIG. 1, a typical oil-filled circuit breaker comprises anoil-containing tank 10 having mounted on it insulating bushes 11 forconductors 14 by which the circuit breaker is connected in the powerline to be protected. From the inner ends of the bushes 11, theconductors 14 lead to a pair of fixed contacts 12 of the circuitbreaker, these contacts being in separate explosion pots 13. When thecircuit breaker is closed, the contacts 12 are connected electrically bya bridging conductor 15 having at its ends rod-like moving contacts 16which project into the explosion pots 13 into contact with the fixedcontacts 12.

The bridging conductor 15 is carried by an operating mechanism includinga downwardly-movable, insulating operating-rod 17. In operation, ondownward movement of the rod 17, the contacts 16 separate from thecontacts 12 and are withdrawn from the pots 13 and during this movementarcs form between the contacts, the energy of the arcs being dissipatedby heating and vaporizing the oil in the explosion pots 13. The vaporsand gases generated assist to prevent the arc from reforming.

According to this invention, there are also providedelectrically-conductive shunts 20 formed from elements as describedabove.

One such element 21 is shown in FIGS. 2 and 3, and it consists of aporous ceramic ring of say 6 inches diameter and 1 inch thickness andhaving a bore of 1.4 inches diameter, the outer and inner edges 22, 23being chamfered and the flat surfaces being metallized but with a gapleft between the edges of the metallization 24 and the chamfers.

A shunt may conveniently comprise a 10-inch stack 25 of such elements 21mounted on an insulating rod 26.

We claim:

1. A shunt for use in an oil-immersed circuit breaker which shunt ispermeable to the oil so that in use on passage of an electric currentthrough the shunt the oil is heated, the shunt comprising a refractoryconglomerate of particles of at least two materials, respectively ofrelatively a high electrical resistivity and a low electricalresistivity, forming an oil-permeable open cellular structure having anapparent porosity of the element between 25 and 30 percent, the poreshaving a mean pore diameter of 1.3 i- 0.1 microns giving acrosssectional area such as to avoid disruptive failure on injection ofsuch a large quantity of energy as would occur in circuit breaking.

2. An oil-immersed circuit breaker having separable electrical contactsand a shunt connected across the contacts and carrying current duringseparation of the contacts, the shunt comprising a refractoryconglomerate of particles of at least two materials, respectively ofrelatively a high electrical resistivity and a low electricalresistivity, forming an oil-permeable open cellular structure having anapparent porosity of the element between 25 and 30 percent, the poreshaving a mean pore diameter of 1.3 i 0.1 microns giving a crosssectionalarea such as to avoid disruptive failure on injection of such a largequantity of energy as would occur in circuit breaking.

3. A shunt for use in an oil-immersed circuit breaker which shunt ispermeable to the oil so that in use on passage of an electric currentthrough the shunt the oil is heated, the shunt comprising a refractoryclay-carbon conglomerate of particles, the conglomerate having thegeneral composition, by weight of the starting materials:

High Resistivity Material Particles of refractory material such ascalcined clay or alumina 40-90% Low Resistivity Material Carbon 0.540%Basic Binder Clay, such as ball clay and/or bentenite 15-50%,

1. A shunt for use in an oil-immersed circuit breaker which shunt ispermeable to the oil so that in use on passage of an electric currentthrough the shunt the oil is heated, the shunt comprising a refractoryconglomerate of particles of at least two materials, respectively ofrelatively a high electrical resistivity and a low electricalresistivity, forming an oilpermeable open cellular structure having anapparent porosity of the element between 25 and 30 percent, the poreshaving a mean pore diameter of 1.3 + OR - 0.1 microns giving acrosssectional area such as to avoid disruptive failure on injection ofsuch a large quantity of energy as would occur in circuit breaking. 2.An oil-immersed circuit breaker having separable electrical contacts anda shunt connected across the contacts and carrying current duringseparation of the contacts, the shunt comprising a refractoryconglomerate of particles of at least two materials, respectively ofrelatively a high electrical resistivity and a low electricalresistivity, forming an oil-permeable open cellular structure having anapparent porosity of the element between 25 and 30 percent, the poreshaving a mean pore diameter of 1.3 + or - 0.1 microns giving across-sectional area such as to avoid disruptive failure on injection ofsuch a large quantity of energy as would occur in circuit breaking.
 3. Ashunt for use in an oil-immersed circuit breaker which shunt ispermeable to the oil so that in use on passage of an electric currentthrough the shunt the oil is heated, the shunt comprising a refractoryclay-carbon conglomerate of particles, the conglomerate having thegeneral composition, by weight of the starting materials: HighResistivity Material Particles of refractory material such as calcinedclay or alumina 40-90% Low Resistivity Material Carbon 0.5-40% BasicBinder Clay, such as ball clay and/or bentenite 3-50%, said clay beingof relatively a high electrical resistivity and said carbon having a lowelectrical resistivity, forming an oil-permeable open cellular structurehaving an apparent porosity of the element not less than 25 percent thepores being of a selected cross-sectional area such as to avoiddisruptive failure on injection of such a large quantity of energy aswould occur in circuit breaking.